The present invention relates to a rotary horizontal table type filter and a carrier structure for such a filter.
Horizontal table type filters generally comprise a carrier structure which is continuously rotated at a constant speed by a suitable device, and a circular filtration table which is mounted on the carrier structure.
More specifically, the carrier structure generally comprises a ring which is driven by a motor, radial gussets or angle plates which are fixed to the ring and circumferential gussets or angle plates which are fixed to the radial gussets. The carrier structure can be constructed of mild steel, for example.
The filtration table comprises a plurality of sectors each including radial walls, one or more bottom walls, circumferential walls defining one or more trapezoidal box sections and an upper wall formed by a sheet of metal with orifices and on which the filter cloth is fixed. The table can be constructed of stainless steel, for example. The bottom of each box section is connected to a vacuum circuit by conduits, which provide for removal of the filtration liquid that passes into the box section through the filter cloth. An example of such a filter is disclosed in French Pat. No. 1,327,693.
The table is usually assembled to the carrier structure by bolts in such a way that the two components are completely fixed relative to each other. More specifically, the walls which form the bottom of the table and which may be configured as trapezoidal troughs or buckets are welded at their periphery to the radial and circumferential walls of the table. The radial walls of the table are mounted in pairs so as to straddle and grip the upper end of the radial gussets of the carrier structure. Each pair of walls are connected together by a first series of bolts and they are also fixed to the corresponding radial gusset by a second series of bolts.
Moreover, the circumferential walls of the table which define the box sections are suppored on and bolted to corresponding circumferential gussets of the structure.
Although filters of this type are in operation, provide a high degree of reliability and permit high production capacity, they may nonetheless have limitations under certain circumstances.
More specifically, it has been observed that the filtration table may be deformed outside tolerance limits in the operation of large-scale filters, or with increasing severity in certain conditions of performance under which the filter is used, for example with higher temperatures or tonnages, or when materials with a very high coefficient of expansion are used for the filter.
Thus, it has been found that the table, although flat when cold, may become slightly spherical during the operation thereof, with the center of the table rising and the outward portion being displaced downwardly. The sectors forming the table thereby tend to pivot about the drive ring of the carrier structure.
Such deformation is troublesome, and can complicate or interfere with extraction of the filter cake. For example, when the filter cake extraction operation is performed by an endless screw which is disposed in a horizontal position at a well defined height above the table, the result of any increase in the height of the screw above the table is to reduce the quantity of cake extracted, while any reduction in the above-mentioned height gives rise to the danger of the filter cloth being torn by the screw. It is therefore necessary for the position of the extraction screw to be adjusted at relatively frequent intervals to accomodate for such changes in its position.
In addition, the stresses generated by deformation of the table may result, inter alia, in accelerated wear of the filter or may cause fractures of the welds, in particular between adjacent sectors, and that in turn may cause the table to leak, with the danger of the filtrate passing between the table and the structure.